This is a story of Annie Flynn,the future daughter of Phineas Flynn and Isabella Garcia-Shapiro , and her step-cousin, Bob Fletcher, the future son of Ferb Fletcher and Lily Smith, on summer vacation. Every day the kids embark on some grand new project, which annoys Bob's controlling sister, Bessie, who always tries to bust them. Then Annie has a pet cat named Katy who acts as a secret agent (Agent K), to fight villains unlike her ally Perry the Platypus, who fought only one villain (Dr. Heinz Doofenshmirtz) 20 years ago.

It also has several musical numbers, but some of it belong to Phineas and Ferb.